Ignore system files when using FileInputFormat

The FileInputFormat adds files that start with an underscore ( _ ) when the given input file is a directory during the creation of InputSplits.

MapReduce jobs create an empty `_SUCCESS` file and a `_logs` directory. These are not relevant when reading the files.

This error also caused empty InputSplits.
FileInputFormat is the base class for most InputFormats.
Hadoop has a smiliar mechanism, which is more advanced (they have a Filter-system that supports multiple filters to be registered at the file system). My implementation does use the same filter as their FileInputFormat. This approach also allows to chain filters.